A microlens is an optical element in either CCD or CMOS imager. By putting a microlens on top of a photodetector, the strength of generated electrical signals is increased, as is the sensitivity of the image sensor. Methods of making microlens in a cost effective way have been described in U.S. Pat. No. 5,324,623, granted Jun. 28, 1994 to Tsumori et al. for Microlens forming method, and U.S. Pat. No. 6,163,407, granted Dec. 19, 2000, to Okazaki et al., for Microlens array and method of forming same and solid-state image pickup device and method of manufacturing same. The technology described therein includes forming a lens shape in photoresist, followed by etching pattern transfer to final lens material. The methods to form lens shape on photo-resist include photo-resist reflow, or direct lens shape patterning from grayscale mask. The photo-resist reflow method has a fill factor limitation, and the grayscale mask is normally fabricated by e-beam photolithography, e.g., as used by Canyon Material. Inc., which is both slow and expensive. A process technology to fabricate grayscale mask in both a cost effective way by using silicon rich oxide (SRO) as a light attenuating material has been developed. By forming the SRO layer into the lens shape and by putting the film on the reticle plate in the litho tool, the pattern printed on the photo-resist surface will have the lens shape accordingly base on the specifications of the litho tool, e.g., a 5× reduction rate on the tool. The photolithographic method of fabricating prototype SRO based grayscale mask is disclosed in U.S. patent application Ser. No. 11/657,326, filed Jan. 24, 2007, for Method of Making a Grayscale Reticle Using Step-Over Lithography for Shaping Microlenses, which is incorporated herein by reference, wherein SRO is deposited on quartz substrate, and patterned and etched into the microlens array.
In order for the grayscale mask to be used in a fabrication process, it has to be made on a reticle plate having required alignment features so that it may be mounted, and used on a litho tool. A reticle used in a litho tool is normally made out of square quartz plate while conventional IC processing tools are designed to handle round shape wafers. Fabrication of a SRO-based grayscale mask on a square quartz plate in an IC processing line is not feasible without major renovation to many of the process systems. One way to accomplish this is to use a transfer technology which allows a completed SRO grayscale mask, formed on a round wafer, to be placed on a square quartz plate.
Wafer bonding technology is used in the IC industry as one method to form a silicon on insulator (SOI) device, which in turn facilitates high-performance CMOS technologies. In wafer bonding technology, the proton, or H2 ion implantation, at the beginning of a process, and cleaving or splitting at the end of a process, is required in the Smart Cut® method.